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Abstract:
The perception of 3D static shapes from 2D images is strongly influenced by surface shading cues (Brewster, 1847; Ramachandran, 1988; Yamane, 2008). Most research on biological motion perception has focused on the influences of 2D motion and form cues and binocular disparity, while the influence of surface shading has only rarely been addressed. We discovered a new visual illusion, where the light source direction flips the perceived walking direction, indicating a lighting from above prior in biological motion perception. METHODS: Our experiments builds on a bistable biological motion stimulus that shows a walker from the front (Vanrie, 2004). Instead of dots we used volumetric conic elements placed at the limb centers. The elevation of the light source direction was varied systematically between 180 degrees (above) to 0 degrees (below). Two stimuli were compared that were derived from the same 3D walker walking either towards or away from the observer. Participants had to report the perceived walking direction. In an additional control experiment we investigated the influence of individual features on the illusion, by removing the gradual shading from all limb segments except for selected subsets. RESULTS: The light source position has a strong influence on the perceived walking directions (for walking away: F(16,176)=178.9,p< 0.01; towards: F(16,176)=154.3,p< 0.01), where illumination from below results in a flip of the perceived walking direction by 180 deg. The control experiment shows that effect is mainly driven by the shading gradients of the forearms and the thighs. CONCLUSION: Similar to the perception of static shapes, biological motion perception depends on a lighting-from-above prior. We were able to reproduce this effect can by extending a neural model for biological motion perception (Giese, 2003) by an additional shading pathway, which analyzes gradual shading variations within surface elements, while suppressing contrast edges at the borders.
